In the wake of the quick development in the technical field of an {character pullout}C, a memory card and the like, requirement to make them thinner and smaller in size is increasingly more severe. Such a requirement is directed also to a semiconductor device employable for an {character pullout}C and/or a memory card.
Referring to FIG. 1 illustrating a cross section of a semiconductor device packaged in a plastic package available in the prior art, a plurality of leads 603 (The leads 603 are arranged on a semiconductor device chip in parallel to one another.) are adhered to the top surface 601a (the surface under which monolithic electronic components are produced and actually the bottom surface in FIG. 1) of a semiconductor device chip 601 by employing an adhesive tape e.g. polyimide resin type 602. Each of the leads 603 is bonded with each of bonding pads 104a produced on the top surface of the semiconductor device chip 601, by an Au wire 604. The semiconductor device chip 601 provided the leads 603 is molded in a plastic mold e.g. a mold of polyimide resin 605, remaining the top surface of the ends 603a of the leads 603 and the bottom surface 601b of the semiconductor device chip 601 unmolded. Although the thickness of the plastic mold 605 is in the range of 0.3-0.4 mm in the vertical direction, it is marginal on the side surface of the semiconductor device chip 601. This is for the purpose to make the thickness of a finished semiconductor device packaged in a plastic package thin and to make the horizontal area thereof small.
After turning the finished semiconductor device packaged in a plastic package upside down, the ends 603a of the leads 603 are soldered with foot prints 610 arranged on a printed board 610a. In this manner, electric connection is secured between the end 603a of the lead 603 and the foot prints 610 by means of a solder lump 611.
Since a semiconductor device chip 601 is mounted on a printed board in an upside-down position, and since the bottom surface thereof is not covered by a plastic mold e.g. a mold of polyimide resin 605 in the prior art, the thickness of a finished semiconductor device packaged in a plastic package is sizably reduced.
The foregoing semiconductor device packaged in a plastic package available in the prior art is, however, involved with a drawback wherein the solder lumps 611 connecting the end 603a of the leads 603 and the foot print 610 arranged on the printed board 610a is inclined to be cracked, resulting in unsatisfactory mechanical and electric connection between the end 603a and the foot print 610. This is of course a parameter to degrade the reliability of a semiconductor device packaged in a plastic package from electrical and mechanical viewpoints.
This adverse phenomenon is assumed to be caused by large difference in coefficient of thermal expansion between a semiconductor device chip and a printed board and by weak connection between a lead 603 and a foot print 610. In other words, the length of a lead 603 is designed to be short for the purpose to reduce the horizontal area of a semiconductor device packaged in a plastic package. Therefore, it is difficult for the lead 603 to absorb a stress derived from a difference in coefficient of thermal expansion between a semiconductor device chip and a printed board. In addition, a small area of the end 603a of a lead 603 which is caused by the design in which the length of the lead 603 is made short, causes to make the connection strength weak between the end 603a of a lead 603 and the foot print 610.
In the foregoing environments, development of a semiconductor device packaged in a plastic package, wherein the connection between a lead and a printed board is strong, thereby the semiconductor device packaged in a plastic package is reliable from electrical and mechanical viewpoints is strongly required.